In the context of relativity: causality concerns the questions which
events can, in principle, cause which other events and which events are
two far apart for one to influence the other. In special relativity,
nothing, no moving object, no information, no influence can move faster
than light. Thus, no event can influence another if the two events
happen too far apart for light to travel between from the first event
to reach the location of the second event in time. In other words,
light propagation determins the causal structure of space-time (cf. light-cone). Models or theories respecting this causal structure are themselves called causal - an example are the relativistic quantum field theories.

In general relativity,
the cosmic speed limit light speed is only defined locally: In a
side-by-side race, no object, no influence can overtake a light signal.
This, too, leads to a causal structure, to strict rules which event can
influence which other event. As gravitydeflects and
delays
light signals,
matters are more complicated than in special relativity, but it's still
true that the causal structure is completely determined by how light
propagates in the space-time in question.

causal sets

Approach to the problem of finding a theory of quantum gravity: In causet models, space-time is composed of elementary building blocks that represent elementary events.

That discipline in physics (and astronomy) dealing with the laws
that govern the motions of heavenly bodies. Originally seen as distinct
from the motions of bodies on earth (see Kepler's laws of motion), it has been a sub-discipline of mechanics ever since Newton derived the cosmic laws of motions from more general mechanical laws.

For most astronomical applications, Newtonian, classical
mechanics works perfectly well, however, as soon as high-precision
measurements or strong gravitational fields come into play, celestial
mechanics is governed by laws of relativistic mechanics derived from Einstein's theory of general relativity.

Celsius

Most European countries use the Celsius temperature scale in
everyday life. Temperatures are given in "degrees Celsius"
(abbreviated as °C). By definition, the zero point of this
scale (0°C) is the melting point of water, while the temperature
100°C corresponds to its boiling point (both parts of the definition
assume the same standard value for air pressure).

Relation to the Kelvin temperature scale used in physics: X degree Celsius are
X plus 273.15 Kelvin, Y Kelvin are Y minus 273.15 degrees Celsius.
In particular, differences in temperature are the same in Kelvin and
in degrees Celsius; the only difference between the two scales is
their choice of zero point.

European research centre for nuclear and particle physics (Centre Européen pour la Récherche Nucleaire - pardon my French), located near Geneva on both sides of the franco-swiss border, founded 1954.

CERN isn't famous just because of particle accelerators like its proton synchrotron, the Large Electron Positron Collider (LEP) and the Large Hadron Collider (LHC) currently under construction, but also as the birthplace of the World Wide Web.

Upper bound for the masses of white dwarfs, in other words: for what low-mass
stars become when
they have used up their nuclear fuel. The first to calculate this upper
bound was the Indian astrophysicist Subramanian Chandrasekhar.

The Chandrasekhar mass is 1.4 times as large as the
solar_mass.
The reason that no white dwarf can have more mass follows from its need to
maintain equilibrium between the gravitational force working towards
further collapse and the interior pressure of the star acting to
prevent collapse. For larger masses, the degeneracy pressure on which a white dwarf's stability
depends is overcome, and further collapse ensues.

charge

On the one hand: a measure of the strength of a force
(action-at-a-distance)
originating from a body, and of how susceptible it is to being
influenced by other bodies via the same force. The most famous example
is electric charge: Electrically charged bodies act on other
electrically charged bodies via an electric force whose strength is proportional to the electric charges of the bodies involved.

It is a characteristic property of charges that they are
conserved; they can neither be created from nothing nor simply disappear. For instance, when a positron with electric charge +1 (in suitable units) and an
electron
with electric charge -1 annihilate to give electromagnetic radiation,
overall charge conservation is satisfied: Before the annihilation, the
sum of the electric charges was 1+(-1)=0, and afterwards, when there is
only uncharged electromagnetic radiation left, it is also zero.

In the context of particle physics, there are more abstract charges not directly connected with forces and interaction, but subject to similar conservation laws.

The changes in the abundances of the different chemical elements that have taken place throughout the history of the universe, mainly in the very early universe during the phase called Big Bang Nucleosynthesis and, from a couple of hundred million years later until today, in the interior of stars (stellar nucleosynthesis).

classical

In physics, the word is used with two meanings. First of all, it
denotes physical models or theories that take into account neither the
effects of Einstein's theories of relativity nor those of quantum physics, for example classical mechanics. However, it is also used to denote models or theories that are not formulated in the framework of quantum physics; in that sense, general relativity is an example for a classical theory.

More information about the different types of singularity can be found in the spotlight text Spacetime singularities.

conservation laws

Some of the most important quantities in physics are conserved :
What they measure can neither be created nor destroyed, and their total
sum is constant over time. Such statements of constancy over time are
called conservation laws.

The most important conserved quantity is energy.
Energy can neither be created from nothing nor simply vanish. If the
energy contained in a system increases, it must be because energy has
been transported into the system (and there is now less energy outside
the system); if the energy decreases, it must be because energy has
been transferred into the system (and there is now less energy outside).

One of the basic postulates of special relativity: The
speed of light in a vacuum is the same for all observers drifting through gravity-free space (more precisely: for all inertial observers. In particular, its value its independent of an observer's motion relative to the source of the light.

continuous

Space as we are used to thinking about it is a continuum or,
equivalently, continuous space: Between every two points, there always
exists an infinity of other points, and every volume can be divided
into smaller and smaller parts without ever reaching a limit.

coordinates

A rule for assigning to each point of a general
space (that is to say: of a line segment, a surface, three-dimensional space or higher-dimensional analogues) or space-time a set of numbers for purposes of identification.

Many readers will know two examples from school: In the case of the
line of real numbers, every point on the line corresponds to a real
number which can be seen as its coordinate. What's important is that
these coordinates reflect neighbourly relations: The number 1 lies
between the number 0 and the number 2, and so does the point
corresponding to it lie between the two points corresponding to 0 and
2. The second example is the usual X-Y-coordinate system (sometimes
called Cartesian coordinates), by which every point in a plane can be
characterized by two numbers: the first its X coordinate value, the
second its Y coordinate value.

The examples reflect an important property of coordinates: To
uniquely identify a point in space, one needs as many coordinate values
as the space has dimensions.

Of the four coordinates defining an event in space-time, three serve to fix its location in three-dimensional space, while the fourth gives the point in time for the event.

The Coriolis force plays an important role in meteorology - from the
point of view of an observer at rest on the surface of the earth, it
explains the deflection of certain wind flows.

Cosmic background radiation

"Electromagnetic echo" of the early universe; first predicted by the
big bang models in the context of general relativity; later, from the 1960s on, observed with radio telescopes.

The cosmic microwave background contains important information about
the properties and the earliest history of the universe. For instance,
it can be used to deduce whether space is curved or Euclidean; more information about this can be found in the spotlight text Cosmic sound.

Is is quite likely that singularities are artefacts resulting from the fact that Einstein's theory does not take quantum effects into account, and that they will be absent in a more complete theory of quantum gravity.
Yet even if you leave aside quantum theory, and stay strictly within
the framework of Einstein's theory, it is likely that most
singularities are, if not absent, then at least well-concealed:

The hypothesis of cosmic censorship states that, whenever a body
collapses so completely as to result in the formation of a singularity,
a black hole will be formed so that the singularity will be hidden behind the horizon, and thus completely unobservable for anyone outside the black hole.

At the present time, this hypothesis is unproven. Indeed, there are
some counterexamples, but they describe idealized situations which are
not likely to tell us anything about the real world. Finding a proof
that, for all realistic collapse situations, there is indeed cosmic
censorship, is one of the great open problems of general relativity
research.

Highly energetic particles reaching the earth from the depths of space, mainly consisting of protons and light atomic nuclei.

cosmic time

Measure for the progress of the evolution of an expanding universe such as that of the big bang models. It corresponds to time as measured by clocks that are at rest relative to the expanding space, and that have been set to zero at the very beginning, the time of the hypothetical big bang singularity. Synonym: Age of the universe.

In the big bang models,
an inherent tendency of space to accelerate or decelerate its
expansion. From observations, it seems that our own cosmos has a
cosmological constant that leads to a slight acceleration of its
expansion.